As is well known, an image sensor is an apparatus for sensing a light beam reflected from an object to generate image data. An image sensor fabricated by using complementary metal oxide semiconductor (CMOS) technology, for example, is called a CMOS image sensor.
Generally, the CMOS image sensor includes a plurality of unit pixels, including a light sensing element and a plurality of transistors. The light sensing element, such as a photodiode, senses the incident light beam and generates photoelectric charges corresponding to the amount of the incident light beam sensed. The transistors perform switching operations to control transfer of the photoelectric charges.
FIG. 1 is a circuit diagram showing a conventional unit pixel 10 contained in a CMOS image sensor. Here, a reference symbol ML represents a load transistor for controlling a current that flows via an output node NO of the unit pixel 10. The unit pixel 10 includes a photodiode 12 and four transistors: a transfer transistor MT, a reset transistor MR, a drive transistor MD and a select transistor MS. Reference numerals TX, RX and SX denote control signals to turn on and off the transistors MT, MR and MS, respectively.
The photodiode 12 senses an incident light to generate photoelectric charges. The transfer transistor MT, coupled between the photodiode 12 and a sensing node NS, transfers the photoelectric charges to the sensing node NS. The reset transistor MR, coupled between a power terminal VDD and the sensing node NS, transfers a reset voltage level from the voltage source to the photodiode 12 and the drive transistor MD.
The drive transistor MD, whose drain is coupled to the power terminal VDD, amplifies a voltage level at the sensing node NS to output an amplified signal. The select transistor MS, coupled between the drive transistor MD and the output node NO, performs a switching operation to output the amplified signal as image data via the output node NO.
FIG. 2 is a cross-sectional view showing a conventional CMOS image sensor, where a field oxide layer 21, a transfer transistor 22 and a photodiode 23 are formed on a semiconductor substrate 20. An insulating layer 24 and a color filter 25 are sequentially formed on the entire semiconductor structure. Additionally, an over coating layer (OCL) 26 for planarization is formed on the color filter 25, and a micro-lens 27 is formed on the OCL 26.
In such conventional sensors, since the color filter 25, the OCL 26 and the micro-lens 27 are formed by using separate photoresist layers, an exposure step and a development step are carried out for each layer. It is, therefore, difficult to selectively replace a defective layer among the stacked photoresists. Additionally, the optical transmittance is low in such multilayer CMOS image sensors and residual products that affect sensor performance can occur during fabrication.